278 research outputs found
Optical properties of arrays of quantum dots with internal disorder
Optical properties of large arrays of isolated quantum dots are discussed in
order to interpret the existent photoluminescence data. The presented theory
explains the large observed shift between the lowest emission and absorption
energies as the average distance between the ground and first excited states of
the dots. The lineshape of the spectra is calculated for the case when the
fluctuations of the energy levels in quantum dots are due to the alloy
composition fluctuations. The calculated lineshape is in good agreement with
the experimental data. The influence of fluctuations of the shape of quantum
dots on the photoluminescence spectra is also discussed.Comment: 7 pages (twocolumn) LATEX, 6 Postscript figure
Musical chairs in a boreal peatland: how permafrost thaw reverses successional processes
The current climate trends indicate amplified high latitude warming. Boreal peatlands can be found within those high latitudes and have important functions hydrologically, ecologically and also in terms of carbon cycling. Peatland’s are wetlands that have accumulated more than 40 cm of peat and can range from minerotrophic fens to ombrotrophic bogs. Naturally, a rich fen can be converted to a bog once groundwater sources are cut off by Sphagnum spp. In areas underlain by discontinuous permafrost, landscape changes are occurring particularly rapidly as the permafrost there is sensitive to both vertical and horizontal thaw. The purpose of this thesis is to determine whether permafrost thaw can lead to the reversal of successional pathways, converting a bog into a fen by increasing hydrological connectivity. The study was conducted in the northern boreal peatlands of the Northwest Territories sampling from rich fens, poor fens and collapse scar bogs, while examining how different levels of hydrological connections (isolated, ephemerally connected or fully connected) impacted species community. We concluded that bogs are resilient to increased connectivity due to permafrost thaw, as they did not increase in richness with increased connectivity. Fens meanwhile showed great variation in richness with increased connectivity. Although there were no whole sale differences in species community, rich fen species have begun encroaching into the fully connected collapse scar bog, providing support for the reversal of the normal autogenic pathway. If no whole scale changes could be found in species composition, pH and nitrate both increased with connectivity, which indicates that we are seeing differences in water chemistry. Finally, flooding simulations were used to determine whether frequency of inundation of a fully connected bog by fen water was predicting soil water chemistry ii and species composition. The data indicates that flooding alone cannot explain the presence of rich fen species and we speculate that nutrients are being released with permafrost thaw. Boreal peatlands are unique habitats and these changes in permafrost may eventually lead to the conversion of these wetlands and the loss of ecosystem functions
Exciton lifetime in InAs/GaAs quantum dot molecules
The exciton lifetimes in arrays of InAs/GaAs vertically coupled quantum
dot pairs have been measured by time-resolved photoluminescence. A considerable
reduction of by up to a factor of 2 has been observed as compared
to a quantum dots reference, reflecting the inter-dot coherence. Increase of
the molecular coupling strength leads to a systematic decrease of with
decreasing barrier width, as for wide barriers a fraction of structures shows
reduced coupling while for narrow barriers all molecules appear to be well
coupled. The coherent excitons in the molecules gain the oscillator strength of
the excitons in the two separate quantum dots halving the exciton lifetime.
This superradiance effect contributes to the previously observed increase of
the homogeneous exciton linewidth, but is weaker than the reduction of .
This shows that as compared to the quantum dots reference pure dephasing
becomes increasingly important for the molecules
Probing energy barriers and quantum confined states of buried semiconductor heterostructures with ballistic carrier injection: An experimental study
A three-terminal spectroscopy that probes both subsurface energy barriers and
interband optical transitions in a semiconductor heterostructure is
demonstrated. A metal-base transistor with a unipolar p-type semiconductor
collector embedding InAs/GaAs quantum dots (QDs) is studied. Using
minority/majority carrier injection, ballistic electron emission spectroscopy
and its related hot-carrier scattering spectroscopy measures barrier heights of
a buried AlxGa1-xAs layer in conduction band and valence band respectively, the
band gap of Al0.4Ga0.6As is therefore determined as 2.037 +/- 0.009 eV at 9 K.
Under forward collector bias, interband electroluminescence is induced by the
injection of minority carriers with sub-bandgap kinetic energies. Three
emission peaks from InAs QDs, InAs wetting layer, and GaAs are observed in
concert with minority carrier injection.Comment: 11 pages, 4 figures, submitted to Physical Review
Non-linear exciton spin-splitting in single InAs/GaAs self-assembled quantum structures in ultrahigh magnetic fields
We report on the magnetic field dispersion of the exciton spin-splitting and
diamagnetic shift in single InAs/GaAs quantum dots (QDs) and dot molecules
(QDMs) up to = 28 T. Only for systems with strong geometric confinement,
the dispersions can be well described by simple field dependencies, while for
dots with weaker confinement considerable deviations are observed: most
importantly, in the high field limit the spin-splitting shows a non-linear
dependence on , clearly indicating light hole admixtures to the valence band
ground state
Correlated Photon-Pair Emission from a Charged Single Quantum Dot
The optical creation and recombination of charged biexciton and trion
complexes in an (In,Ga)As/GaAs quantum dot is investigated by
micro-photoluminescence spectroscopy. Photon cross-correlation measurements
demonstrate the temporally correlated decay of charged biexciton and trion
states. Our calculations provide strong evidence for radiative decay from the
excited trion state which allows for a deeper insight into the spin
configurations and their dynamics in these systems.Comment: 5 pages, 3 figures, submitted for publicatio
Measurement of cortical thickness asymmetry in carotid occlusive disease
Despite being considered an important anatomical parameter directly related to neuronal density, cortical thickness is not routinely assessed in studies of the human brain in vivo. This paucity has been largely due to the size and convoluted shape of the human cortex, which has made it difficult to develop automated algorithms that can measure cortical thickness efficiently and reliably. Since the development of such an algorithm by Fischl and Dale in 2000, the number of studies investigating the relationship between cortical thickness and other physiological parameters in the brain has been on the rise. There have been no studies however that have validated cortical asymmetry against known vascular anatomy. To this aim, using high-resolution MRI, we measured cortical thickness and volume in the primary motor (M1) and primary visual (V1) cortex in patients with unilateral, high-grade carotid occlusive disease (n = 29, age = 74 ± 10 years). These regions were selected based on the hypothesis that there will be thinning of the cortical thickness of M1 in the territory supplied by the occluded carotid artery, whereas V1 will show no asymmetry since its blood supply is provided by unaffected posterior arteries. To test for an effect of handedness, cortical thickness and volume were also measured in healthy volunteers (n = 8, age = 37 ± 13 years). In patients, we found thinner cortex in M1 on the occluded side (mean = 2.07 ± 0.19 mm vs 2.15 ± 0.20 mm, p = 0.0008) but no hemispheric difference in V1 (1.80 ± 0.17 mm in occluded vs 1.78 ± 0.16 mm in unoccluded, p = 0.31). Although the mean cortical volume of M1 in the occluded hemisphere was also lower, the difference did not reach statistical significance (p = 0.09). Similarly, in healthy controls, the results showed no hemispheric asymmetry in either cortical thickness or volume in either region (p \u3e 0.1). To test for an orientation bias in the method, the analysis was repeated with images flipped from neurological to radiological orientation. While the algorithm did not yield identical results for the two orientations, the effect did not alter the findings of the study. These results provide a method for within-subject validation of a pathophysiological effect of carotid occlusive disease on the human cortex and warrant further investigation for underlying mechanisms
Hybrid MBE-CBE Growth and Characterization of Al 0.48 In 0.52 As on InP(100) for avalanche photodiode applications Motivation
International audienceIn this work, we demonstrate the epitaxial growth of high quality, low strain and low background doping of Al0.48In0.52As at 500°C on Fe-doped semi-insulating InP(100) substrate by using hybrid MBE-CBE technique. The precursors that were used are: solid aluminum, solid indium, TriMethylIndium (TMIn) and thermally cracked arsine. Using Nomarski, we observed smooth surfaces for the as grown layers. High-Resolution X-ray Diffraction (HR-XRD) in the vicinity of the (004) reflexion shows a lattice mismatch in the range -137 to 127ppm. The carrier density of undoped layers, obtained by Hall measurement at room temperature, is as low as 3E+15 cm-3 which is three orders of magnitude lower than the identical layers grown by organometallics sources. Photoluminescence (PL) for Al0.48In0.52As at low temperature (LT) shows a good optical quality. The quality and purity of the alloys grown here are compatible with high performance APD for optical communication
Ensemble interactions in strained semiconductor quantum dots
Large variations in InxGa1-xAs quantum dot concentrations were obtained with simultaneous growths on vicinal GaAs [001] substrates with different surface step densities. It was found that decreasing dot-dot separation blueshifts all levels, narrows intersublevel transition energies, shortens luminescence decay times for excited states, and increases inhomogeneous photoluminescence broadening. These changes in optical properties are attributed to a progressive strain deformation of the confining potentials and to the increasing effects of positional disorder in denser dot ensembles
Spatially Resolved Visible Luminescence of Self-Assembled Semiconductor Quantum Dots
Ensembles of defect-free InAIAs islands of ultrasmall dimensions embedded in AIGaAs have been grown by molecular beam epitaxy. Cathodoluminescence was used to directly image the spatial distribution of the quantum dots by mapping their luminescence and to spectrally resolve very sharp peaks from small groups of dots, thus providing experimental verification for the discrete density of states in a zero-dimensional quantum structure. Visible luminescence is produced by different nominal compositions of InxAI(1-x)As-AIyGa(1-y)As.NRC publication: Ye
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